利用連續變頻聲納對海底沉積物之淺層反射係數的估算研究

Abstract

過去對於水域底質之判定多採用傳統岩芯取樣的方法，結果除沒有效率外，對區域的代表性也不佳。本次研究目的為測試使用聲波傳遞的方法，用來估算海床上和其下方淺層沉積物的反射係數值，進而判斷調查地方沉積物的顆粒大小分布。使用工具是連續變頻聲納(chirp sonar)系統，為EdgeTech公司1990年出產的Xstar系統，音鼓載具型號是SB-216S，作業使用頻段為2- 10kHz，工作地點在苗栗竹南外海。利用聲波反射能量來計算反射係數值，需考慮各介質中對能量的擴散、衰減等因素，研究中為了計算聲波在傳播過程中減少的能量，考慮了音束束寬傳播和聲波波長的效應關係，去求取反射係數解，這過程所得到的校正值和根據現場海床樣本的粒徑分布屬性，並利用Hamilton(1970)的實驗值尚稱吻合。本研究方法進一步用來計算海床次層的反射係數，用鑽孔資料做比對，結果顯示對海床下的聲波地層介面反射係數的估算偏差量，越往深處誤差可能會隨訊號強弱而改變。利用聲波去計算反射係數值，必須考量近場域壓力場測量值的代表性是否充分，或需建立一合理的聲場校正程序方能有效。參考測區附近的岩心資料對比本研究得出的各層反射係數值，驗證結果符合實際的沉積物淺層分佈的情形。

In order to know the physical properties of seabed sediments, people traditionally adopted to practical grabbing the samples of seabed sediments and analyzed it directly. But there are dozens of drawbacks like inefficient for vast area and unrepresentative for region as a whole. This study aimed at looking for a method that is more efficient to acquire the reflection coefficients of seabed sediments and further to each interface of sediment layers beneath. The tool used is the geological chirp sonar called which the Xstar system composed of a transducer vehicle SB-216S produced by the EdgeTech in 1990. The survey was conducted in Chu-Nai offshore area and operated bandwidth in 2-10kHz. Draw upon acoustic reflection power to calculate the reflection coefficients that would contemplate on the factors of energy spreading and attenuation when acoustic waves transport the downwards in different mediums. In order to estimate the amount of energy loss, to precisely find the values of reflection coefficient, the spreading effect on beamwidth as well as the Fresnel zone effect on reflected energy have been considered. The calibration ratio is therefore obtained, and the value is compatible with the study adopted local seabed sediment sample to amend the reflection coefficient according to conclusion of Hamiltion(1970). The method we study is feasible to calculate reflection coefficient on seabed layers further beneath, and the result would illustrate more deviated in the deep when the singal/noise ratio is significantly decreased. If we utilized acoustic power to calculate reflection coefficient, we shall first considering the representative of Near-field pressure measurement is adequate or not. Or we have to set up a rationally calibration process of acoustic field. At last, we compare the reflection coefficients of 3 acoustic interfaces in shallow layers beneath seabed with the core drilling data nearby to attest what we calculate is in keeping with reality.